1. Field of the Invention
The present disclosure relates to a high current solid target for radioisotope production at a cyclotron using a metal foam.
2. Description of the Related Art
In general, radioisotope refers to any isotope which emits radiation such as alpha, beta, and gamma rays. Isotope means any element which has the same chemical properties as a common element but differs in atomic weight, and may be largely classified into nuclear reactor nuclides and accelerator (cyclotron) nuclides. Nuclear reactor nuclides are usually used for cancer treatment and accelerator nuclides are usually used for cancer diagnosis.
Because accelerator nuclides among the nuclides are carrier-free and have high specific activity unlike nuclear reactor nuclides, they may significantly reduce exposure of patients to radioactivity due to its decay by electron capture or emission of positrons, and may obtain quality images for diagnosis, accelerator nuclides are usually preferred in nuclear medicine. Furthermore, the positron emitting nuclides are used in positron emission tomography (PET) or single photon emission computed tomography (SPECT) for study of metabolism in the human body and diagnosis of cancer, cardiological disorders, and various diseases caused by nervous system disorders.
With respect to isotopes for medical diagnosis and treatment, their applications are increasing and their demand is on the rise. The demand for medical cyclotron nuclides in Korean nuclear medicine is increasing by 10% every year. However, 80% or more of the demand is dependent on imports, leading to numerous studies on increasing production.
Radioisotopes are produced by irradiating protons or neutrons on stable isotopes. An apparatus or device with which protons or neutrons may be irradiated on stable isotopes refers to a target, and the target device receives high energy protons accelerated by a cyclotron, which in turn induce nuclear reactions in stable isotopes, to change the material state of the stable isotopes such that they may be transformed into radioisotopes.
Target devices for production of radioisotopes include solid, liquid, and gas state targets according to stable isotopes used. 18F and 123I are produced from gas targets while 201Tl, 103Pd, and 67Ga are produced from solid targets. In particular, most of the metal-based materials are used as a solid target for production of SPECT isotopes.
FIG. 1 illustrates the configuration and principle of the solid target device. As illustrated in FIG. 1, the solid target device includes a solid target plate 1, a stable isotope 2 plated on the target plate, a cooling unit 3 in which cooling water flows in order to cool the target, and an irradiation station 4 to which proton beam is irradiated. A proton beam for production of isotopes is produced by an accelerator called a cyclotron, and the protons were used to cause nuclear reactions to transform stable isotopes into radioisotopes for production.
FIG. 2 illustrates the structure of a conventional solid target device. The diameter of a part through which a proton beam is introduced is within about 10 mm, and the beam is irradiated by using a wobbler device such that the beam produced by a cyclotron may have as wide and uniform of a distribution as possible. Tilted targets are generally used as a solid target in order to irradiate a high current beam. An area onto which the beam is irradiated may be enlarged by using a tilted target, and it is possible to irradiate a high current beam because the plate thickness for stable isotopes in the target may be significantly reduced.
Protons accelerated by a cyclotron are characterized by a drastic drop in energy according to the density of a material. Because the energy which has been diminished in this way is generated as heat, higher cooling efficiencies are required as the beam current of irradiation increases. When heat is generated, a target does not maintain the intrinsic solid state of metal and is vaporized by a proton beam therefore the degree of vacuum of the cyclotron is reduced. As a result, not only may the performance be degraded, but the isotope productivity may also be reduced due to a lowered nuclidic purity in most cases, arising from the inability to maintain an energy band sufficient to irradiate onto a target material. For this reason, it is very important to cool the target surface of a solid target. Therefore, it is necessary to enhance the cooling efficiency of a solid target in order to secure the stability of production yield, reduce irradiation time, and increase the quantity of an isotope produced during irradiation of a high current proton beam.
A heat transfer coefficient by a fluid flow may be defined as h in the following Formula 1.
                    h        =                              Q            A                    ⁢                                    1                              Δ                ⁢                                                                  ⁢                T                                      ⁢                                                  [                          W              ⁢                              /                            ⁢                              m                2                            ⁢              K                        ]                                              [                  Formula          ⁢                                          ⁢          1                ]            
In Formula 1, Q is a heat quantity transferred, A is a heat transfer area, and ΔT is a temperature difference. As seen from Formula 1, the cooling efficiency becomes high as the heat transfer coefficient increases. Therefore, a heat transfer area must be increased or a temperature difference between two media must be enhanced in order to enhance the cooling efficiency.
Conventional approaches for enhancing the cooling efficiency have been largely divided into the two directions: increasing the cooling flow rate in order to maintain a constant temperature difference and enhancing the cooling efficiency by increasing the irradiated area to increase a heat transfer area.
Because an interval for which fluids are not flowing due to frictional force between a metal surface and cooling water is created by methods for increasing the cooling flow rate, a flow channel must be narrowed for flow of cooling water in order to prevent this. In order to maintain the cooling efficiency by limitation of a limited flow channel, measures to increase the pressure of cooling water must be taken and as a result, problems such as leakage of cooling water and leakage into a vacuum unit occur.
According to methods for increasing the irradiated area, it is difficult to perform chemical treatments by increasing the plated area of a stable isotope, there is a limitation in size arising from a problem of beam uniformity in an accelerator, and expensive stable isotopes must be inevitably used for a larger area.
Thus, the present inventors have conducted studies to enhance the cooling efficiency of high current solid targets for radioisotope production, discovered that a high current solid target for isotope production which attaches a metal foam to the rear surface of the solid target plate exhibited excellent cooling performances such as increasing the amount of the proton beam current irradiated on the solid surface by 1.5 to 2-fold compared to conventional planar-type solid targets, and completed the present invention.